Steve Bannon, former White House chief strategist and chairman of Breitbart News, at Monday's conference on countering violent extremism in Washington.

WASHINGTON — Rex Tillerson’s beleaguered four-month effort to end the isolation of Qatar just hit a new snag: Stephen K. Bannon.

President Donald Trump’s former chief strategist, with whom Tillerson clashed over personnel and policy decisions during his time in the Trump administration, emerged on Monday during a think tank event in Washington to drum up support for Saudi Arabia and the United Arab Emirates, two Gulf powers Tillerson is actively pressuring to ease their isolation of Qatar, home to a key US military base.

During a panel hosted by the Hudson Institute, Bannon gave a full-throated defense of the list of demands Saudi Arabia and the United Arab Emirates placed on Qatar in June to end the diplomatic crisis — an ultimatum that Tillerson for months has considered an unrealistic impediment to resolving the standoff.

“I took a very hard line on that. I thought the UAE and the Egyptians and the Kingdom of Saudi Arabia had a well thought-through plan,” he added.

Bannon’s unusual foray into the technicalities of US-Gulf diplomacy comes weeks after his former colleagues at SCL Social Limited registered as foreign lobbyists for the UAE and one day after Tillerson arrived in Qatar in search of a breakthrough to the crisis.

Gulf analysts said Bannon’s intervention adds another obstacle to Tillerson’s efforts to resolve the conflict, which have been complicated by Trump’s tweets in support of the blockade and his son-in-law’s backchannel communications with Saudi Arabia.

“Undoubtedly Bannon’s lobbying on behalf of the UAE against Qatar makes it far more difficult for Tillerson to project the Trump administration as an impartial mediator,” said David Ottaway, a Gulf expert at the Wilson Center. “The Saudis and Emirates believe Bannon reflects Trump’s visceral bias against Qatar because of its support for the Muslim Brotherhood and Hamas.”

The State Department declined to comment on Bannon’s remarks.

Since the beginning of the summer, Tillerson has struggled to resolve the crisis between the US Gulf allies, a rift that resulted from long-simmering differences over Qatar’s relations with Iran, funding for Islamist groups, and funding of Al Jazeera, a state media outlet that gives a platform to a variety of voices in the Arab world, including those opposed to the governments of the UAE and Saudi Arabia.

Shortly after Trump’s first foreign trip to the Saudi capital of Riyadh in May, the Gulf countries cut diplomatic ties with the tiny energy-rich country and imposed an air, sea, and land blockade.

From the perspective of Tillerson, and Secretary of Defense James Mattis, the dispute threatens to destabilize the region and jeopardize the US air base in Qatar, which is home to the largest concentration of US military personnel in the Middle East.

Saudi Arabia and the UAE insisted their concerns with Qatar were due to its financing of terrorist groups, a charge many Middle East observers say Saudi Arabia and the UAE are guilty of, too. But key White House officials, including Bannon and Trump’s son-in-law Jared Kushner, supported the Saudi position. Trump initially voiced support for the blockade, but has since said it’s time for the dispute to be resolved.

Bannon, carving out an alternative view, said on Monday that the pressure brought to bear on Qatar amounted to one of the most successful foreign policy achievements of Trump’s young presidency. “I think the single most important thing that’s happening in the world is the situation in Qatar,” he told the audience.

Though US officials have previously denied that the US gave Arab countries advance support for the move, Bannon claimed the plan sprung out of the summit of Muslim countries Trump convened in Riyadh in May. “It was looked at as an opportunity to be seized,” Bannon said on Monday. “Qatar finally had to be called to account for their continual funding of the Muslim Brotherhood, their continual funding of Hamas.”

Bannon, who is currently the executive chairman of Breitbart News, was asked about his associations with SCL Social Limited, a firm that recently signed a $330,000 contract with the National Council of UAE for an anti-Qatar influence campaign.

SCL has the same owners and leadership team as Cambridge Analytica, which Bannon worked for as vice president and had a stake in worth $1 million to $5 million at the time he entered the White House. Despite those connections, Bannon told the audience “that's a company I have nothing to do with.”

Mandel Ngan / AFP / Getty Images

Trump joins dancers with swords in Riyadh on May 20, 2017.

The notion that Bannon still holds sway with the president even after his departure from the White House is prevalent internationally. In recent weeks, Bannon has met privately with Wang Qishan, a top Chinese government official, and Sheikh Mohammed bin Zayed al Nahyan, the influential crown prince of Abu Dhabi. Bannon “still talks to the president,” according to the New York Times, “even if the White House plays down the frequency of those contacts.”

The perception that Tillerson can be circumvented by other Trump confidantes more sympathetic to their position poses serious risks for Tillerson and the State Department, experts said.

“The Saudis and their allies believe that they have the inside track to the White House through Kushner and Bannon and they can ignore the State Department on issues like Qatar and Yemen,” said Bruce Riedel, a Brookings Institution scholar and former CIA officer. “So far they are right.”

Simon Henderson, a Gulf expert at the Washington Institute for Near East Policy, said Bannon’s intervention raises questions about whether the nation’s top diplomat speaks for the president’s true positions. “To Tillerson’s frustration, Bannon’s involvement in the anti-Qatar campaign will be interpreted by Riyadh and Abu Dhabi as implicit support by President Trump for their continuing hardline.”

Jeremy Singer-Vine contributed to this report.

John Hudson is a foreign affairs reporter for BuzzFeed News and is based in Washington, DC.